Stepped frequency radar

ABSTRACT

A multi-port junction is fed with a frequency-stepped source and has one of its ports connected to an antenna that can serve either as a transmit-and-receive antenna or as a receive antenna only, with the outputs of the multi-port junction being used to estimate a complex reflection coefficient for each frequency of interest. The subject system requires no IF stages, down-conversion mixers or oscillators, and therefore may be provided adjacent each antenna at low cost. An embodiment involving co-located separate transmit and receive antennas is used to minimize the power requirements for the multi-port junction, whereas in a third embodiment, an array of transmit/receive antennas is used, fed by the same RF source but in which digitally-controlled phase shifters are used for beam-forming purposes.

RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 11/632,027filed Jan. 9, 2007 now U.S. Pat. No. 7,570,200 entitled SteppedFrequency Radar, which is a 371 of PCT/US06/01075 filed Jan. 12, 2006which claims benefit of 60/643,542 filed Jan. 13, 2005, the contents ofwhich are incorporated herein by reference. This Application is relatedto U.S. application Ser. No. 11/110,263 filed Apr. 20, 2005, and U.S.patent application Ser. No. 11/035,311 filed Jan. 13, 2005, the contentsof both are incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to stepped frequency radars and more particularlyto the use of multi-port junctions in combination with circuits forestimating a complex reflection coefficient to provide radar functions.

BACKGROUND OF THE INVENTION

As described in the aforesaid U.S. application Ser. No. 11/110,263 byMatthew A. Taylor, Kevin S. Bassett, and Paul E. Gili, entitled “Methodand Apparatus For Transmission Line and Waveguide Testing”, and Ser. No.11/035,311 by Joshua Niedzwiecki, entitled: “Reduced ComplexityTransmission Line And Waveguide Fault Tester”, both assigned to theassignee hereof and incorporated herein by reference, multi-portjunctions are used in combination with a module for generating a complexreflection coefficient from the outputs of the multi-port junction inorder to establish in one embodiment the existence of faults in atransmission line and the severity thereof. In these applications, areflectometer having a frequency source that is stepped from onefrequency band to the next provides a complex reflection coefficientprofile with frequency for each of the faults or discontinuities in thetransmission line.

When the estimated complex reflection coefficient is processed by anInverse Fourier Transform, then the frequency domain information isconverted to time domain information. The time domain information isthen converted to distance to a fault or range to a fault in thetransmission line.

Algorithms described in the Taylor patent application and in theNiedzwiecki patent application include various algorithms for estimatingcomplex reflection coefficients from respectively a six-port junctionand a four-port junction in which the output signals from the multi-portjunctions are functions of both the signal source and the returnedreflected signal.

It will be appreciated that when multi-port junctions are utilized forreflectometers, gone are the usual IF stages, heterodyning, mixers andoscillators which are in general used to convert received signals tobase band where the processing occurs.

The same heterodyning techniques are used in current stepped frequencyradars in which received signals are down-converted by an intermediateIF stage to an IF frequency. The result of the down-conversion is thendigitally sampled to bring it down to base band where one is able toexamine the differences between what is transmitted and what isreflected.

The problem with such stepped frequency radars is the cost of such IFstages, which include expensive oscillators and mixers.

By way of further background, frequency-swept radars have been used asground penetrating radars in which sub-surface objects are to beidentified, such as land mines, pipes, voids in concrete and othersubterranean objects. There are a wide variety of time domainreflectometers and systems that develop their information byascertaining the range to the discontinuity by detecting, for instance,round trip travel times.

Where there are a number of ultra wideband ground penetrating radarsthat use swept frequencies, their resolution and the ability to identifysubsurface objects leaves something to be desired. Others have suggestedusing pulsed radars as ground penetrating radars. The problem with suchground penetrating radars or through-the-wall radars that use pulsetechniques is that, as one gets higher in frequency, one cannotadequately control the leading and trailing edges of the pulses so thathigh fidelity resolution is not possible.

It would be desirable to have a high fidelity frequency resolution inwhich a high fidelity map of the reflection coefficient of the returnedenergy across frequency is determined. However, if one were to attemptto use time domain reflectometry techniques for a ground penetratingradar, one can only achieve limited fidelity to, for instance, fullycharacterize the frequency response of the reflecting objects.

It is therefore impossible utilizing traditional time domainreflectometers to obtain a high fidelity radar map of the reflectioncoefficients across frequency.

While stepped frequency radars have been used in the past that arefrequency agile, it is important to be able to ascertain with highfidelity what is happening in the main lobe of the radar using sensitivetechniques that require phase coherence of the radar itself. How oneaccomplishes frequency stepping while maintaining phase coherence isindeed a problem and one that heretofore has required expensiveequipment to be able to generate phase-coherent transmitted radiationand to be able to analyze the returns based upon samples of thetransmitted radiation.

Moreover, in the past and as described in U.S. Patent Application SerialNO. PCT/US04/20116 by Paul Zemany entitled Dual FrequencyThrough-the-wall Motion Detection and Ranging Using Difference-BasedEstimation Technique, filed Sep. 14, 2004, assigned to the assigneehereof and incorporated herein by reference, two-color radars have beenused to be able to detect motion of individuals behind a wall in aso-called through-the-wall system. In this system, CW signals of twoalternating different frequencies are used to detect the presence ofmoving individuals behind a wall. Moreover, when multiple frequencybands are available, one is able to locate not only the fact of a movingindividual, but also the location of the moving individual. Moreover,when more and more radars surround, for instance, a building, one cantriangulate to more accurately estimate the position of the individual.One therefore needs an inexpensive frequency-stepped CW radar for theseapplications.

Such through-the-wall systems are extremely useful for fire and policefor commercial applications as well as for the military to be able todetect enemy combatants or soldiers behind a wall or within a building.

As will be appreciated, several through-the-wall applications requirefrequency-stepped radars and for this reason one would like to develop arelatively inexpensive, simplified frequency-stepped radar for thesepurposes.

SUMMARY OF INVENTION

It has been found that one can use the aforementioned frequency domainreflectometers of Matthew A. Taylor and Joshua D. Niedzwiecki toascertain the complex reflection coefficient, not of a transmission linebut rather of anything that is within an antenna beam when, rather thancoupling a transmission line to a test port of a reflectometer using amulti-port junction, one attaches an antenna.

As described in the aforementioned Taylor and Niedzwiecki inventions,one can estimate the complex reflection coefficient as a function offrequency and thereby obtain a reflection coefficient frequency profile.Further, one can use the complex reflection coefficient profile toderive distance to the reflections, thereby to provide rangeinformation.

It has been found that one can use the complex reflection coefficientprofile that results through the use of a multi-port junction fed by avariable-frequency signal source to obtain frequency profiles at variousfrequencies which, when compared with pre-stored profiles, enable one toidentify the object reflecting the transmitted signal.

The stepped-frequency radar can use a single antenna for transmittingout a CW radar beam and for receiving the returns, or can be adapted toa situation in which one antenna transmits the beam and a co-locatedantenna receives the returns. In this latter case, since the receivedsignals are a great deal lower in amplitude as compared with thetransmitted signal, one need not have a high power multi-junctioncircuit at the receive side. This decreases the cost of the overallsystem, since one can use low-power multi-port junctions.

In a still further embodiment of the invention, one can use thesemulti-port junctions at each antenna or pair of antennas in an array.This permits the rendering of a three-dimensional map of the objectsreflecting the energy, along with an identification of what the objectsare.

In either case, a variable frequency signal source is connected to oneport of a multi-port junction, with the other port of the multi-portjunction coupled to an antenna to transmit out at least a portion of thesignal source output. Because of the use of the multi-port junction, aportion of the transmitted signal is mixed with a portion of thereceived signal and with appropriate processing results in theaforementioned complex reflection coefficient frequency profile.

As will be appreciated, the stepped frequency radar can operate with alow-cost RF source that does not contain any mixing circuitry orhigh-speed sampling circuitry that can be very costly and can limitperformance.

The stepped-frequency approach of the subject invention also allows foraccurate spectral characterization of whatever is in the field of viewof the transmit antenna. Moreover, the bandwidth of the frequency sweepis variable so that the depth resolution can be easily adjusted for eachapplication with no increase in system cost. With a pulse radar system,this can only be achieved by using very short-duration impulses or verysharp pulse edges that require expensive, high-performance signalsources and high-speed data acquisition circuits. Even so, adequateperformance above 1 GHz is problematical. Thus, in certain instances,for instance at 18 GHz, the required ultra-short pulses are impossibleto generate.

Moreover, when the antennas are arrayed, the subject invention permitsobtaining high spatial resolution through beam forming. In thisapplication, the radar design is implemented in an array-based systemwhere multiple transmit and receive antennas are arrayed.

As mentioned above, the invention is capable of operating with separatetransmit and receive antennas, although it can also combine the transmitand receive functions into the same antenna to improve performance andreduce cost.

Note that in the beam-forming applications the beam may beelectronically steered using digitally controlled phase shifters so thatthe radar can scan a given field of view. Moreover, with specializedsignal processing algorithms, one can use the subject system toadaptively track a specific location of interest, for instance, wherethere is a buried object. This enables the signal-processing algorithmnot only to locate each object but also to examine the details of eachobject with the highest signal-to-noise ratio possible.

As will be appreciated, this technique can be used to locate buriedobjects that can be easily detected and characterized using the complexreflection coefficient and the subject processing algorithms.

Thus, because the subject invention can be implemented at low cost whilemaintaining high performance, it lends itself to many applications. Oneapplication is the aforementioned ground penetrating radar application.In this case, underground objects such as land mines, tunnels or pipes,can be detected with a high degree of accuracy. Structural analysis canbe performed by examining the internal components of the structure. Forexample, concrete foundations are often examined usingground-penetrating radar to examine the location and structure of therebar and to detect any pockets that may cause stress. Anotherapplication for the subject invention is to use it for theabove-mentioned through-the-wall radar, which can be used by police orthe military to ascertain if people are in a building or behind a wall.

Rather than attempting to use time domain reflectometers for suchpurpose, the subject frequency domain reflectometer operation permitsbetter resolution and better ability to identify different features ofreflecting objects.

In one embodiment, the subject invention uses a bank of microwavedetector circuits, each connected to an antenna and each being fed by asingle RF signal source.

In one embodiment these detector circuits are four-port junctions, witheach four-port junction having two outputs containing two orthogonal RFsignals that are functions of both the source signal and the returnedsignal.

These outputs are coupled to power detector circuits, where the poweroutput of each is measured over a fixed period of time. The process isrepeated for each frequency in a sweep, with the output powermeasurements amplified and sampled by an analog-to-digital converter andthen processed by a signal processor to obtain the aforementionedcomplex reflection coefficient profile.

The processor uses the sample power measurements out of each detectorcircuit and specialized signal processing algorithms to estimate thecomplex reflection coefficient, S₁₁(f), as seen from each antenna foreach frequency step in the sweep.

This reflection coefficient, also known as the two-way propagationcoefficient, S₂₁(f), defines how each signal is attenuated and phaseshifted for each frequency step.

Once the stepped process is complete, a transfer function of thepropagation medium is defined as a function of frequency. Signalprocessing algorithms are then use to process this spectral response inspace, time and frequency domain to detect and classify buried objects.

In one embodiment, what is provided is a so-called single elementfour-port-based radar with a single transmit/receive antenna. Here aportion of a frequency-stepped RF signal from a signal source is coupledout of the antenna and received reflections are passed through a numberof 90° quadrature hybrids as well as a 0° splitter to provide theabove-mentioned outputs. These two outputs are then used to estimate thecomplex reflection coefficient.

In the second embodiment, the RF signal source is directly coupled to atransmit antenna, which is located adjacent a receive antenna. In thisembodiment, a portion of the RF signal source is divided down or splitand is combined with the reflected energy returned to a receive antennafrom objects within the transmit beam of the transmit antenna. Thepurpose of this embodiment is so that a maximum amount of transmit powercan be radiated, while the multi-port junction used in generating thecomplex reflection coefficient need not be a high-power device andtherefore inexpensive.

In a third embodiment in which the transmit and receive antennas are thesame antenna, an array of such antennas can be provided in which amulti-port junction is provided for each transmit/receive antenna.Digital phase shifters may be used between the RF source and each of themulti-port junctions associated with each transmit/receive antenna so asto provide beam-forming functions.

In summary, a multi-port junction is fed with a frequency-stepped sourceand has one of its ports connected to an antenna that can serve eitheras a transmit-and-receive antenna or as a receive antenna only, with theoutputs of the multi-port junction being used to estimate a complexreflection coefficient for each frequency of interest. The subjectsystem requires no IF stages, down-conversion mixers or oscillators, andtherefore may be provided adjacent each antenna at low cost. Anembodiment involving co-located separate transmit and receive antennasis used to minimize the power requirements for the multi-port junction,whereas in a third embodiment, an array of transmit/receive antennas isused, fed by the same RF source but in which digitally-controlled phaseshifters are used for beam-forming purposes.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the subject invention will be betterunderstood in connection with a Detailed Description, in conjunctionwith the Drawings, of which:

FIG. 1 is a diagrammatic illustration of a single-element,four-port-based radar, illustrating the use of the four-port junctionfed by an inexpensive frequency-swept RF signal source having one of itsports coupled to a transmit/receive antenna, with the output of thefour-port junction being used to generate a complex reflectioncoefficient frequency profile useful in identifying the range andcharacteristics of a reflecting object;

FIG. 2 is a diagrammatic illustration of an embodiment involving aseparate transmit antenna and receive antenna, illustrating that ratherthan feeding the transmit antenna through the multi-port junction, aseparate transmit antenna is used, with the receive antenna beingcoupled to an inexpensive multi-port junction;

FIG. 3 is a diagrammatic illustration of a multi-element four-port-basedradar in which an array of transit/receive antennas, each coupled to itsown four-port junction, is used for beam forming in which eachmulti-port junction has a common signal source and digitalphase-shifters interposed between the signal source and respectivemulti-port junctions for beam-forming purposes; and,

FIG. 4 is a flow chart showing an algorithm for determining the distanceto objects and selected characteristics of the objects to enabledetection of the type of object returning energy, illustrating the useof object templates and a feature database, in which one comparesextracted features to a feature database for different objects and oneclassifies objects and reports the location and angle and range from theradar.

DETAILED DESCRIPTION

Referring now to FIG. 1, in one embodiment of the subject invention, afour-port junction 10 has a port 12 connected to an RF signal source 14.Four-port junction 10 also has an antenna port 16 coupled to a combinetransmit/receive antenna 18 as illustrated.

There are two output ports 20 and 22 at which output power is developed,which is applied to an analog-to-digital converter 24, in turn coupledto a central processor 26 that is in turn coupled to a user interface28.

Four-port junction 10 includes a 90° quadrature hybrid 30, which has aninput port 32 coupled to port 12 and an input port 34 coupled to port16. Hybrid 30 also has an output port 36 coupled to a 0° power divider38 that has output ports 40 and 42 coupled to respective 90° quadraturehybrids 44 and 46. The outputs of respective hybrids 44 and 46, namelyports 48 and 50, are coupled to the input ports to a 90° quadraturehybrid 60 having output ports 62 and 64 coupled to power detectors 66and 68 that are in turn coupled to output ports 20 and 22. Note thathybrid 44 has an output port 70 coupled to ground through a load 72,whereas hybrid 46 has ports 74 and 76 coupled to respective loads 78 and80 to ground.

In operation, an RF source signal at port 12 is coupled through hybrid30 to port 16 and thence to transmit/receive antenna 18. Antenna 18receives reflected energy from an object in the lobe of the antenna andcouples this reflected energy back into port 34, where it is transmittedto hybrid 44 over line 82. Hybrid 44 then couples a portion of thisreflected energy to hybrid 60, along with a divided-down portion of theRF source signal that comes from divider 38.

What is therefore available at power detectors 66 and 68 is a portion ofthe RF signal source mixed with a portion of the reflected signal.

Because of divider 38, a portion of the RF signal source passes throughhybrid 46 and then to hybrid 60, where it appears in the output of powerdetector 68. Hybrid 60 mixes the divided-down output of the RF sourcewith a portion of the reflected signal that is available from hybrid 66.

What is therefore available at the outputs of power detectors 66 and 68are 90°-out-of-phase signals that can be used, after analog-to-digitalconversion, by the central processor to calculate the complex reflectioncoefficient for the particular frequency to which the RF source istuned.

Central processor 26 computes from the complex reflection coefficient atleast the range to the reflecting object and indeed other spectralcharacteristics of the reflecting object, such that user interface 28can be used to develop a three-dimensional map of the area surveyed bythe radar.

What makes this a stepped-frequency radar is the fact that the RF signalsource is stepped in frequency over a number of frequency bands andfrequencies so that, as will be described, a complex reflectioncoefficient profile can be generated as a function of frequency.

The above describes the use of a stepped-frequency radar in which asingle transmit/receive antenna is used.

Referring now to FIG. 2, a situation is presented in which a transmitantenna 100 is used in combination with a co-located receive antenna102.

Here, a splitter 104 splits off a small portion of the signal from RFsource 14 and applies it to a different type of four-port junction 110,which has output ports 112 and 114 coupled to analog-to-digitalconverter 24 to permit central processor 26 to derive the complexreflection coefficient from these outputs.

What will be seen is that the reflected energy is collected at receiveantenna 102, where it is coupled to port 116 of a 90° hybrid 118, whereit is mixed with a divided-down RF source signal at port 119 from a 0°power divider 120 having its input port 122 connected to splitter 104.The other portion of the divided-down output from splitter 120 isapplied to an input port 124 of a 90° quadrature hybrid 126, with theoutputs of hybrids 118 and 126 being applied to a further 90° quadraturehybrid 130.

Here it will be seen that respective hybrids 126 and 118 have outputs132 and 134 coupled to inputs 136 and 138 of hybrid 130. Moreover, theoutputs of hybrid 130 are connected to power detectors 140 and 142connected respectively to ports 112 and 114 as illustrated.

Here it will be seen that the majority of the energy from the RF sourceis coupled by splitter 104 to transmit antenna 100. Only a small portionof the energy from RF source 14 is coupled to four-port junction 110 sothat the reduced power signal exists at port 122 of splitter 120.

What will be appreciated is that since the energy that enters port 122of four-port junction 110 is a much-divided-down portion of the RFsource, then the four-port junction need not be provided with any heavycurrent-carrying capability and therefore can be made relativelyinexpensively.

As in the case of FIG. 1, the output power available at output ports 112and 114 are 90° out of phase, with portions of the RF signal source andthe reflected energy available at the outputs of these power detectors.

Referring now to FIG. 3, a situation is depicted in which one requiresan array of antenna elements and in which one seeks to perform beamforming for the frequency-stepped radar. Here, a single RF source 14 iscoupled through digital phase shifters 140 to a plurality of four-portjunctions 150, 160, 170 and 180 as illustrated. Four-port junction 150is coupled to transmit/receive antenna 152, whereas the remainder of thefour-port junctions 160, 170 and 180 are coupled to respectivetransmit/receive antennas 162, 172 and 182.

The configuration of the four-port junctions is identical to thatillustrated in FIG. 1.

As can be seen, the RF signal inputs to these four-port junctions foreach of the illustrate circuits is respectively illustrated at inputports 154, 164, 174 and 184.

Each of the respective four-port junctions has its own analog-to-digitalconverter, here illustrated at 156, 166, 176 and 178, with the four-portjunctions being of sufficiently small size and cost so that each of thetransmit/receive antennas of the array may be provided with its ownfour-port junction.

Phasing is accomplished through the digital phase shifters, whichphase-shift the output of RF signal source 15 to provide for theaforementioned beam forming.

As can be seen from FIG. 1, in the first embodiment of the subjectinvention, the radar is controlled through a user interface. When theradar is activated by a user, the central processor controls the RFsource and sets it to transmit a sinusoid at a frequency f, defined bythe user. This signal is fed to the input of the four-port junction andsome energy is coupled out of the antenna, with the rest coupled throughthe four-port junction to both power detectors.

The signal that gets transmitted out of the antenna propagates throughthe medium it is directed to. In a ground penetrating radar application,the antenna would be pointed to the ground and the transmit sinusoidwould propagate through the ground. At each discontinuity in material,electromagnetic properties, some of this transmitted energy will bereflected, for example, at interfaces between the ground and the airbetween the soil and a land mine.

A percentage of each transmitted signal is phase shifted and radiatedback to the radar antenna and the signal propagates into the four-portjunction and is coupled with the RF source signal to the input of eachpower detector. The two power detectors measure the power of thestanding wave as seen at the detector input. This power measurement istaken by scaling and sampling the voltage out of each detector with ananalog-to-digital converter. Because the power of the signal does notchange in a stationary environment while the RF source is transmittingat a set frequency, the output voltage of each detector is a DC signal.Therefore, the analog-to-digital converter at the detector output doesnot need to be high speed. The analog-to-digital converter samples eachdetector output and integrates over a fixed time window to reduce thenoise bandwidth. Once these measurements are taken and stored at thecentral processor, the RF source is stepped to the next frequency (f₂)and the process is repeated. This continues until the entire bandwidthdefined by the operator has been swept.

Referring to the FIG. 2 embodiment, the circuit is similar to that ofFIG. 1 but the RF source power is split. The four-port circuit thenoperates identically to the circuit of FIG. 1, and power measurementsfrom the four-port junction are measured across the entire frequencyband.

As mentioned before, the purpose of this type of circuit that uses aseparate transmit and receive antenna is to limit the power that isinputted to the four-port junction, thereby making possible the use oflower-power four-port junctions.

Referring to FIG. 3, each of the four-port junctions functions is asspecified in FIG. 1. However, in this case the RF source power is splitand phase-shifted to multiple four-port junctions. Each of thesecircuits then operates identically to the FIG. 1 embodiment except thatthe detection and classification algorithm change. The phase shiftersare controlled by the central processor and are used to form and steer atransmit/receive beam using the antenna array and the four-portcircuits. Of course, the shape and size of the array can be varieddepending on the application.

Power measurements from each four-port junction are measured across theentire frequency band just as before, but they are also measured at auser-defined set of angles of arrival. The signal processing algorithmin the FIG. 3 embodiment is similar to those employed in FIG. 1, withthe exception that in the FIG. 3 embodiment there is an added spatialdimension used for further target discrimination and betterclassification.

Note that when the antennas are arrayed, they can provide spatialdiversity. One other discriminator that can be used is polarization.Using antennas with orthogonal polarization adds an additional set ofequations for the same amount of unknowns, improving the probability ofdetection and reducing the probability of false alarms.

Referring now to FIG. 4, in one embodiment the processing includes firstthe estimation of the complex frequency response as a function of angleof arrival and polarization is illustrated at 200. As illustrated at202, one then converts the returns to distance, whereas at 204, onedetects objects using object templates that have been previously storedas illustrated at 206.

Having detected objects, one can extract or impose features asillustrated at 208 from a pre-stored feature database 210.

As illustrated at 212, one can compare the extracted features to afeature database for different objects and, as illustrated at 214, onecan classify the objects and report the location in terms of angle andrange from the radar.

It will be appreciated that the algorithms in the aforementioned Taylorand Niedzwiecki patent applications for respectively the four-port andthe six-port junctions can be used to estimate the complex reflectioncoefficient, with the algorithms used varying between the six-port andthe four-port embodiments.

Thereafter, one can use the Taylor Modified Inverse Fourier Transform toconvert the frequency domain information into time domain information,thereby eliminating the problems associated with conventional timedomain reflectometers and the lack of ability to produce short enough orwell-defined enough pulses.

One uses the Taylor algorithms to take into account both phase andattenuation in the transmission medium from the antenna to the objectthat is reflecting the radiation back. Thus, the subject system takesadvantage of the Modified Inverse Fourier Transform to more accuratelydetect amplitude peaks from which distance or range can be determinedand to reject noise or multiple reflections.

Moreover, the Taylor techniques can be invoked to eliminate ghosts, withthe use of the complex reflection coefficient also used to eliminateghosts and false returns, unlike the use of the absolute magnitude ofthe reflection coefficients of some prior reflectometers.

While the present invention has been described in connection with thepreferred embodiments of the various figures, it is to be understoodthat other similar embodiments may be used or modifications or additionsmay be made to the described embodiment for performing the same functionof the present invention without deviating therefrom. Therefore, thepresent invention should not be limited to any single embodiment, butrather construed in breadth and scope in accordance with the recitationof the appended claims.

1. A method for adapting a frequency domain reflectometer utilizing amulti-port junction having two input ports, one input port coupled to afrequency source, into a radar, comprising the steps of: coupling aradar antenna to the other of the multi-port junction inputs; generatinga complex reflection coefficient from the outputs of the multi-portjunction; and, performing radar functions using the complex reflectioncoefficient derived from the outputs of the multi-port junction.
 2. Aradar, comprising: a frequency source; a multi-port junction having twoinputs and a number of outputs, one of said inputs coupled to saidfrequency source; a radar antenna coupled to the other of said inputports; and, a radar processor coupled to said outputs for performingradar functions.
 3. The radar of claim 2, and further including aplurality of said multi-port junctions and antennas, with said antennasbeing arrayed.
 4. The radar of claim 3, wherein said signal source iscoupled to each of said plurality of multi-port junctions.
 5. The radarof claim 4, and further including phase delays interposed between saidsignal source and respective multi-port junctions to provide beamsteering.
 6. The radar of claim 2, and further including a transmitantenna coupled to said signal source, and a power reducing unit coupledbetween said signal source and said one input to reduce the input signalpower thereto.
 7. The radar of claim 2, wherein said processor includesan estimator for estimating a complex reflection coefficient from theoutputs of said multi-port junction.
 8. The radar of claim 7, whereinsaid processor includes an Inverse Fourier Transform for converting saidcomplex reflection coefficient to distance.
 9. The radar of claim 8,wherein said processor includes a detector coupled to the output of saidInverse Fourier Transform to detect objects having characteristics closeto object template characteristics.
 10. The radar of claim 9, whereinsaid processor includes a feature extractor coupled to said detector toselect those detected objects having features close to pre-selectedfeatures.
 11. The radar of claim 10, and further including an objectclassifier for classifying objects based on said detected objects andsaid selected features and for reporting the location of said classifiedobjects.
 12. The radar of claim 11, wherein said reported locationincludes angle and range from said radar's antenna.
 13. The radar ofclaim 2, wherein the frequency of said frequency source is variable. 14.The radar of claim 2, wherein said frequency source is frequencystepped.